Irritable bowel syndrome is a common disease affecting approximately 12% of the adult population of the United Sates. Evidence suggests irritable bowel syndrome sufferers have altered Gl flora. Probiotics are living microorganisms that exert health benefits beyond inherent nutrition. When ingested, they colonize the gut. Lactobacillus GG (LGG), unlike other Lactobacilli, consistently colonizes the intestine. Nine studies to date, one using LGG, have examined the role of probiotics in the treatment of IBS with conflicting results. Using an Internet-based, randomized double-blind placebo-controlled clinical trial design, which will allow us to enroll a large number of patients from all over the country, we will, as a primary aim, (1) determine if IBS patients given LGG will report an overall response to therapy. Secondary aims will be to (2) determine if LGG will lead to lower scores on the IBS Frequency and Bothersomeness Questionnaire for specific symptoms of IBS; (3) determine if LGG use will result in higher IBS-specific quality of life scores; (4) test for bias by mode of administration of questionnaires by comparing responses to Internet-based questionnaires with those obtained by standard interview in a subset of subjects brought into clinic; and (5) ensure that IBS patients can become colonized with LGG after taking it by performing stool cultures on this subset of subjects. This study is the main component of a 5-year career development plan. The plan, which integrates mentored research with continued formal coursework, is designed to culminate in my becoming an independent investigator and a leader in the research areas of probiotics, gut flora, non-antibiotic approaches to infection, and use of the Internet-based study design. This study benefits public health in, several ways. First, it establishes the effectiveness or ineffectiveness of a treatment for a common disease for which there are few acceptable therapies. Second, it lays the ground work for the future use of the Internet-based clinical trial for other studies of irritable bowel syndrome, and for future probiotic studies.